In the field of display technology represented by liquid crystal displays (referred to as LCDs), due to improved resolution, increased display size and low resistance wiring required in integrating driver circuits in a display device, wires and electrodes, such as gate lines, and data lines as well as gate electrodes, source electrodes and drain electrodes in thin film transistors (referred to as TFTs), which are made of metal with low resistance characteristics (e.g., copper) have already been applied to the display devices. However, when this kind of metal (e.g., copper) is used for wiring or serves as an electrode, the metal is likely to diffuse into adjacent circuit elements or semiconductor layer in the thin film transistor, resulting in deterioration of performance of pixel elements or thin film transistors. Therefore, the metal atoms (e.g., copper atoms) contained in the wires or electrodes need to be prevented from spreading around or diffusing into the adjacent elements or semiconductor layer.